Mike's Wildlife Articles of the Osa
Courtesy of  El Sol de Osa The Osa Peninsula's Newspaper

Osa Safari: The Bushmaster,
by Mike Boston of Osa Aventura

It is huge, rare and shrouded in mystique. No other snake in the New World inspires such reverence and dread within us than the mighty bushmaster. At over 4 meters, it is the largest viper in the world. And one of the deadliest!

Myth and legend surround this snake wherever it is found, and this is reflected in its many vernacular names: the “Sucurucu”, the Brazilians believe, can extinguish fires, and will suck the milk from cows and sleeping women; the “Matabuey”, the Costa Ricans say can kill oxes; “Cascabel muta” inspired Carolus Linnaeus, the founder of modern biological nomenclature, to name it Crotalus muta, the silent rattlesnake. Early explores to the New World returned with fanciful accounts of this monstrous serpent laying waste to whole mule trains.

So impressed was Francois-Marie Daudin, a late 18th century herpetologist, that he assigned the bushmaster to its own genus, Lachesis, named after one of the three fates in Greek mythology: Clothos, the spinner of life’s thread; Lachesis, the drawer of lots, chose the length of one’s thread; and Atropos made the final cut. Lachesis muta, the silent arbiter of one’s fate!

So much for the myth: what of the real bushmaster? Well it is a truly remarkable snake in every way: reported to grow to 4.25 meters, the bushmaster is by far the largest viper in the world. Only the king cobra of Asia and the common taipan of Australia, both members of the cobra family (the Elapidae), oust it from the title as largest venomous snake in the world. It is the only viper in the New World to lay eggs (oviparous); all the others bear live young (viviparous). And, what few records there are of bites from this snake, suggest an 80% mortality rate among humans, making the bushmaster the most deadly snake in the Americas.

The bushmaster is rare, though - perhaps fortunately so! Its narrow habitat requirements confine the bushmaster to undisturbed, lowland primary rainforest. In this it differs from its soul mate in terror, the terciopelo, whose cosmopolitans habitat tastes bring it regularly into close proximity to people. So for all its fame, the bushmaster is rarely seen, whereas everybody is familiar with the terciopelo.

Bushmasters, like terciopelos (and all other vipers), rely on stealth and camouflage to surprise and ambush their prey. But, unlike the terciopelo, the bushmaster has a very specific dietary habit: both young and adults eat almost exclusively mammals, from mice to rats, agoutis and opossums. Adult terciopelos will take rats and opossums, in addition to frogs and lizards. But their young feed only on lizards and frogs.

The bushmaster and all of its New World cousins belong to a sub-division of the viper family, the Crotalinae, or pitvipers. They are so named because they posses a pair of pits, one on each side of their heads, between their nostrils and eyes. These pits function as infrared heat sensors, enabling these vipers to detect the body heat of their prey. Indeed, these pits are extremely sensitive to temperature changes (to small fractions of a degree) and in effect function as a second eye, allowing these remarkable snakes to see the world in the infrared spectrum as well as in the visible spectrum of light.

The venom of the bushmaster, and all other vipers, is a complex cocktail of toxic compounds. Its primary function, like the venom from the other main group of poisonous snakes, the cobras and allies, is to immobilize prey. Cobra venom is termed a neurotoxin, and achieves this effect by arresting the nervous function of its prey. Viper venom, termed a haemotoxin, achieves the same result by disrupting the circulation system, among other things. But viper venom has another trick up its sleeve!

All snakes eat relatively large prey, whole. And this presents them with a problem: how to digest the animal before it rots from within. Most snakes solve this problem by possessing powerful digestive systems. But vipers have found a less costly solution: they inject venom that not only kills the prey, but also begins the process of digestion before the prey is even ingested. This digestive quality of viper venom causes rapid tissue damage, however, and makes bites from these snakes particularly nasty. Recovery from a cobra bite will leave little or no visible evidence of the incident. Not so with a viper bite: at best one will be left with unsightly local scaring as a reminder. At worst, amputation!

Drop for drop, the venom of the bushmaster is not as potent as many other venomous snakes, but it is the shear volume of venom and the depth to which its long fangs can inject it, that makes this pitviper so potentially dangerous.

The bushmaster ranges from Nicaragua through the rest of Central America into Colombia and Ecuador in the west, and down into Brazil in the center and east of South America. Within its range, four sub-species are recognized. One of these four sub-species, Lachesis muta melanocephala, is confined to the Osa. It differs from the other sub-species by, among other things, the black coloration of the top of its head – it is known locally as the “Plato Negro”. Another difference is in the plato negro’s temperament. Researchers at Costa Rica’s anti venom institute, the Instituto Clodomiro Picado, have informed me that the Osa’s bushmaster is the most aggressive of them all – the few locals who encountered this snake will attest to this - and responsible for more casualties here than the other sub-species of bushmaster elsewhere in the country.

Ground dwelling, ambush-hunting snakes like the bushmaster, the terciopelo and the boa constrictor rely on cryptic coloration of rhomboidal markings for concealment, both to catch their prey and to hide from predators. When disturbed, these species will remain still, at least initially, relying on their camouflage for protection. Large, active hunting snakes – all non-venomous, or only mildly venomous! - like tiger rat snakes, sipos, mussuranas and cribos react to disturbance by fleeing rapidly. These snakes are also more uniform in color, or at least lack rhomboidal patterning. So if, like me, you delight in the esoteric sport of snake-catching here is a rule of thumb: fast moving, uniformly colored or striped snakes that live on the ground, grab immediately; banded snakes, or rhomboidally colored snakes that remain coiled when approached, hesitate initially, they may be poisonous. This rule falls apart for arboreal snakes, however, so it is wiser to leave all snakes alone!

In appearance, both the bushmaster and the terciopelo look mean and menacing, and some say even malevolent. The terciopelo, being a lance-head pitviper, has pronounced triangular-shaped head. And, as its name implies, the terciopelo has a velvet appearance to its skin. The bushmaster in contrast has a more rounded, rattlesnake-like head, and skin that is covered bead-like scales. The terciopelo, it is said, caries with it an aura of neurotic unpredictability; the bushmaster, an aura of calculated malevolence. Few animals for me embody the spirit and mystique of wild, pristine rainforest like the majestic bushmaster.

A Postscript

Since writing this article, new information about the taxonomic status of the infamous bushmaster has come to my attention. As indicated above, the bushmaster was considered to be one species, with several geographic sub-species. Recent biometric evidence, however, has raised the level of three of these sub-species to specific status: The South American species, Lachesis muta, the Central American bushmaster, L. stenophrys, and the black-headed bushmaster of the Osa, L. melanocephala.

Mike Boston is a biologist, wilderness expedition guide, and the president of Osa Aventura. You can contact him at at info@osaaventura.com